1. Field of the Invention
This invention relates to microcellular foamed or reaction injection molded polyurethane/polyureas prepared from aliphatic or araliphatic diisocyanates wherein the isocyanate group is aliphatically bound.
2. Description of the Prior Art
Reaction injection molded polyurethanes are well-known in the art as described by F. Melvin Sweeney in Introduction to Reaction Injection Molding, Technomics, Inc., 1979. The use of reaction injection molding (RIM) of polyurethanes in the production of structural parts for automotive applications such as fenders, doors and body panels as well as in other applications such as computer housings, office equipment housings, sports equipment and the like is well-known.
In the practice of RIM processes to produce a molding having a compact surface skin, a mold is filled with a highly reactive, liquid starting component within a very short time by means of a high output, high pressure dosing apparatus after the components have been mixed in so-called positively controlled mixing heads. In such "one-shot" processes, the highly reactive starting materials are generally a polyether or a polyester having a plurality of active hydrogen moieties, a polyisocyanate and a chain extender such as a diol or a polyamine as well as other conventional additives such as blowing agents, catalysts, fillers and the like. These are mixed and delivered to a mold within a very short time (generally about 2 to 4 seconds). The mixture is cured to yield a finished product within a short time (generally about 1 to 2 minutes) and thereby produce a polyurethane product. The polyurethane-forming compositions have been the material of choice for RIM applications because their short reaction times allow rapid cure and short cycle times.
Almost all of the commercial production for RIM applications have to date been based upon diphenyl methane diisocyanate (MDI) derivatives. MDI based urethanes have excellent processibility and can be formulated to give a wide range of physical properties. However a major drawback of the MDI based urethanes is poor weatherability. In the presence of ultra-violet light from the sun these urethanes tend to discolor, lose surface gloss, and weaken due to the formation of quinoid structures. The extensive use of stabilizers to protect the MDI-based urethanes against photo degradation have allowed limited use in outdoor applications. For satisfactory performance in actual practice, a light stable topcoat must be applied either by painting after molding or by an in-mold coating process. Both coating processes require additional capital investment and add to the processing cost.
While aliphatic diisocyanates, those with the NCO groups attached to aliphatic rather than aromatic carbon atoms, have been commercially available for many years, and are far less sensitive to photo degradation and weathering than those made with aromatic diisocyanates, these are higher in cost and generally slower in reactivity than the aromatic diisocyanates and therefore have not been used generally in RIM applications. Recently, aliphatic diisocyanates which are lower in cost than previously commercially available aliphatic diisocyanates have become commercially available. These new aliphatic isocyanates, while they contain aromatic moieties, have characteristics typical of aliphatic diisocyanates because the NCO groups are aliphatically bound, i.e., shielded from the aromatic ring by protective methyl groups. These new diisocyanates are disclosed by Arendt et al in Journal of Cellular Plastics, November/December, 1982, pages 376-383, incorporated herein by reference. Other araliphatic diisocyanates are disclosed in U.S. Pat. No. 4,051,166 to Richter et al, incorporated herein by reference. A class of aliphatic diisocyanate is disclosed in U.S. Pat. No. 4,565,835 to Oertel et al, incorporated herein by reference.
Amine-terminated polyethers are disclosed for use in RIM processes by Dominguez in Amine-Terminated Polyether Resins in RIM, Proc. SPI Annu. Tech./Mark. Conf., 28th(Polyurethane Mark. Technol.: Partners Prog.), 312-15 (1984). Amine-terminated polyether resins are disclosed by Dominguez as highly reactive polyether intermediates for use in the RIM process. The use of such amine-terminated polyether resins in RIM processes in combination with aromatic diisocyanates is disclosed in U.S. Pat. No. 4,474,900; U.S. Pat. No. 4,474,901; and U.S. Pat. No. 4,513,133 to Dominguez.
Aliphatic diamines are disclosed in U.S. Pat. No. 4,048,102, to Quock et al, as useful crosslinkers in the preparation of flexible polyurethane foams which can replace 4,4'-methylene bis(2-chloroaniline). Such flexible polyurethane foams are produced primarily utilizing aromatic polyisocyanates, although useful organic polyisocyanates can include aliphatic diisocyanates. Reactive diamines are also disclosed as useful as a reactant in the preparation of a polyurethane molded foam or elastomer product in U.S. Pat. No. 4,384,052, to McBrayer et al and in U.S. Pat. No. 4,210,728, to Patton, Jr. et al. Each of the latter patents describe the preparation of polyurethanes from organic polyisocyanates which can be aliphatic, cycloaliphatic, aromatic or mixtures thereof.
There is no teaching in any of the above references that reaction systems comprising the diamines defined herein, aliphatic or aliphatically bound araliphatic diisocyanates, and amine-terminated polyethers are particularly suitable for RIM processes. Thus the combination of slow reactivity and poor physical properties obtained in the prior art utilizing aliphatic or araliphatic diisocyanates in the RIM process can be overcome by the addition of amine-terminated polyethers with the diamine chain extenders of the invention.